A scanning electron microscope (SEM) detects signal electrons generated when a converged irradiation electron beam irradiates and scans a sample, and displays a signal intensity at each irradiation position in synchronization with a scan signal of the irradiation electron beam, thus obtaining a two-dimensional image of the scan area on the sample surface. An FIB-SEM composite device is known in which a focused ion beam device (FIB) for processing a sample by emitting and scanning a focused ion beam on the sample is combined with an SEM to form an integrated device. In this FIB-SEM, the state of processing with the FIB can be observed with a high level of resolution by the SEM, so that this enables more precise shape processing of a sample such as preparation of a thin film sample for observation with Transmission Electron Microscope (TEM).
In recent years, the FIB-SEM has been widely used for processing samples of wafer shapes at the development site of the most advanced semiconductor devices. In such fields, low-acceleration observation performance with irradiation energy of about 1 keV or less is regarded as important for the purpose of avoiding charging and damage of the sample caused with the electron beam irradiation at the time of SEM observation and obtaining sample information about the top surface. However, with a generally-available SEM, chromatic aberration increases in the low acceleration region, and a high resolution cannot be obtained. In order to reduce this chromatic aberration, it is effective to use a decelerating optical system that allows the irradiation electron beam to pass through the objective lens at a high speed and decelerates and emits the irradiation electron beam just before the sample.
The decelerating optical system is roughly divided into a retarding method and a boosting method depending on a difference in voltage application method. In the retarding method, the objective lens side of the SEM column is kept at the ground potential and a negative voltage is applied to the sample. In the boosting method, a tubular electrode for applying a positive voltage is provided along the inner wall of the inner magnetic path of the objective lens of the SEM, and the sample is set to the ground potential. Both of the retarding method and the boosting method are characterized in that the area of the irradiation electron beam at electron source side from the objective lens is at a higher potential than the sample, and the electric field which is formed by this potential difference and in which the irradiation electron beam decelerates toward the sample is used as a lens field. As the irradiation electron beam passes through the magnetic lens at a high speed, the objective lens aberration can be reduced in a low acceleration range compared with the case where there is no decelerating electric field, and a high resolution can be obtained. Further, the signal electrons generated from the sample are accelerated and converged by this electric field and pass through the SEM objective lens. In such a system, a detector (hereinafter referred to as a detector in the SEM column) disposed in a space set to a higher electric potential than the sample in order to generate a decelerating electric field can efficiently detect the signal electrons having passed through the objective lens.
The improvement effect of resolution and signal electron detection rate with the decelerating optical system depends on the intensity of the decelerating electric field in the vicinity of the sample, and the stronger the electric field, the more effective it is. In order to improve the resolution and detection efficiency, the potential difference for forming the decelerating electric field may be increased. However, when the decelerating optical system is applied to the SEM, the axial symmetry of the electric field distribution with respect to the optical axis of the SEM becomes important. If the distribution of the decelerating electric field becomes nonaxisymmetric due to a structure placed around the SEM column or an inclined sample such as an FIB column, the observation performance of the SEM is remarkably deteriorated.
In the field of semiconductors, when a wafer-shaped sample is processed with FIB, the sample surface is arranged perpendicular to the FIB optical axis, and the asymmetry of the electric field formed near the sample as the SEM decelerating electric field is increased. Since there is no influence as described above in a system not employing the ordinary decelerating optical system, the irradiation electron beam and the irradiation ion beam reach in proximity to the intersection point of the SEM optical axis and the FIB optical axis on the sample (hereinafter referred to as a coincident point) without depending on the unevenness of the surface of the sample and the inclination situation. However, when the symmetry of the electric field distribution in the vicinity of the sample is not good, the trajectories of the irradiation electron beam and the irradiation ion beam are bent and are not guided to the coincident point on the sample. In addition, not only the displacement of the trajectory increases but also the beam diameter at the irradiation position increases due to an increase in the asymmetric aberration component, resulting in degradation of the resolution at the time of SEM observation. Furthermore, the orbit of the signal electron is disturbed, making it difficult for the signal electron to reach the detector in the SEM column. As described above, the asymmetric electric field distribution caused by the SEM decelerating electric field is a factor of deteriorating the irradiation system and detection system performance of the SEM and the FIB.
For the above reasons, in the case of obtaining a high resolution with a device configuration such as FIB-SEM which uses a large inclination angle of a sample, a lens system that reduces aberrations by leakage of a magnetic field from an objective lens is often used without applying a deceleration optical system. However, when there is a magnetic field leakage from the SEM objective lens in the vicinity of the sample, the orbit of the isotope ion contained in the irradiation ion beam of the FIB is separated, so that there is a problem in that it is impossible to perform simultaneous observation with the SEM during the FIB processing. The above problem is avoided by applying the boosting method to the objective lens of the SEM having a small magnetic field leakage so as to minimize the asymmetric electric field distribution formed around the sample.
PTL 1 describes an example in which a shield electrode is provided between an annular electrode provided at the tip of the SEM and the FIB optical axis in an FIB-SEM having such SEM mounted thereon. In PTL 1, a change in the position where the irradiation electron beam arrives due to an asymmetric electric field leaking from the end portion of the objective lens is suppressed by applying a voltage to the annular electrode, and changes in the position where the ion beam arrives due to the electric field formed by applying the voltage to the annular electrode are suppressed by the shield electrode.